In optics, demodulation of a phase modulated beam of light can be performed by interference with a mutually coherent (well defined phase relationship), unmodulated reference beam. The output beam, a combination of the phase modulated and unmodulated beams, is passed to a detector. The phase modulation of the modulated input beam appears as an amplitude modulation of the output beam. The intensity of the output beam varies sinusoidally with the phase difference between the phase modulated beam and unmodulated beam. For small phase modulation the operating point of the demodulator is sensitive to the path difference between interfering beams. As a result, relative motion of the interfering beams will produce a phase modulation noise which cannot be distinguished from a true phase signal encoded on the beam. This is a highly undesirable result. However, through non-linear optic means, path independent or adaptive demodulation is possible, as long as the interfering beams remain mutually coherent.